Methods for establishing thermal joints between heat spreaders or lids and heat sources

ABSTRACT

According to various aspects, exemplary embodiments are disclosed of thermal interface materials, electronic devices, and methods for establishing thermal joints between heat spreaders or lids and heat sources. In exemplary embodiments, a method of establishing a thermal joint for conducting heat between a heat spreader and a heat source of an electronic device generally includes positioning a thermal interface material (TIM1) between the heat spreader and the heat source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority of U.S. ProvisionalPatent Application No. 61/881,823 filed Sep. 24, 2013. This applicationis a continuation-in-part of U.S. patent application Ser. No. 13/918,824filed Jun. 14, 2013. The entire disclosures of the above applicationsare incorporated herein by reference.

FIELD

The present disclosure generally relates to thermal interface materials(TIM1), and more particularly (but not exclusively) to methods forestablishing thermal joints between heat spreaders or lids and heatsources.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Electrical components, such as semiconductors, integrated circuitpackages, transistors, etc., typically have pre-designed temperatures atwhich the electrical components optimally operate. Ideally, thepre-designed temperatures approximate the temperature of the surroundingair. But the operation of electrical components generates heat. If theheat is not removed, the electrical components may then operate attemperatures significantly higher than their normal or desirableoperating temperature. Such excessive temperatures may adversely affectthe operating characteristics of the electrical components and theoperation of the associated device.

To avoid or at least reduce the adverse operating characteristics fromthe heat generation, the heat should be removed, for example, byconducting the heat from the operating electrical component to a heatsink. The heat sink may then be cooled by conventional convection and/orradiation techniques. During conduction, the heat may pass from theoperating electrical component to the heat sink either by direct surfacecontact between the electrical component and heat sink and/or by contactof the electrical component and heat sink surfaces through anintermediate medium or thermal interface material. The thermal interfacematerial may be used to fill the gap between thermal transfer surfaces,in order to increase thermal transfer efficiency as compared to havingthe gap filled with air, which is a relatively poor thermal conductor.Most especially in the cases of phase changes and thermal greases, asignificant gap is not required and the purpose of the thermal interfacematerial may be just to fill in the surface irregularities betweencontacting surfaces. In some devices, an electrical insulator may alsobe placed between the electronic component and the heat sink, in manycases this is the thermal interface material itself.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to various aspects, exemplary embodiments are disclosed ofmethods for establishing thermal joints between heat spreaders or lidsand heat sources. Also disclosed are thermal interface materials andelectronic devices including the same.

In exemplary embodiments, a method of establishing a thermal joint forconducting heat between a heat spreader and a heat source of anelectronic device generally includes positioning a thermal interfacematerial (TIM1) between the heat spreader and the heat source. Inanother exemplary embodiment, a method generally includes positioning athermal interface material (TIM1) between a heat spreader and a heatsource of an electronic device prior to curing an adhesive for attachingthe heat spreader to the electronic device. In a further exemplaryembodiment, an electronic device generally includes a lid and asemiconductor device having a normal operating temperature range. Athermal interface material (TIM1) establishes a thermal joint betweenthe lid and the semiconductor device.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a cross-sectional view of an electronic device showing athermal interface material (TIM1) positioned between a heat spreader(e.g., an integrated heat spreader (IHS), a lid, etc.) and a heat source(e.g., one or more heat generating components, central processing unit(CPU), die, semiconductor device, etc.) according to exemplaryembodiments;

FIG. 2 is a view showing a thermal interface material (TIM1) on asurface of a heat spreader (e.g., an integrated heat spreader (IHS), alid, etc.) according to exemplary embodiments; and

FIG. 3 is a line graph showing durometer test results versus temperaturefor a TIM1 according to exemplary embodiments.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

Heat spreaders are commonly used to spread the heat from one or moreheat generating components such that the heat is not concentrated in asmall area when transferred to a heat sink. An integrated heat spreader(IHS) is a type of heat spreader that may be used to spread the heatgenerated by operation of a central processing unit (CPU) or processordie. An integrated heat spreader or lid (e.g., a lid of an integratedcircuit (IC) package, etc.) is typically a thermally-conductive metal(e.g., copper, etc.) plate that rests on top of the CPU or processordie.

Heat spreaders are also commonly used (e.g., as a lid, etc.) to protectchips or board-mounted electronic components often in conjunction with asealed package. Accordingly, a heat spreader may also be referred toherein as a lid and vice versa.

A first thermal interface material or layer(s) (referred to as TIM1) maybe used between an integrated heat spreader or lid and a heat source toreduce hot spots and generally reduce the temperature of the heatgenerating components or device. A second thermal interface material orlayer(s) (referred to as TIM2) may be used between the integrated heatspreader (or lid) and the heat sink to increase thermal transferefficiency from the heat spreader to the heat sink.

The heat source may comprise one or more heat generating components ordevices (e.g., a CPU, die within underfill, semiconductor device, flipchip device, graphics processing unit (GPU), digital signal processor(DSP), multiprocessor system, integrated circuit, multi-core processor,etc.). Generally, the heat source may comprise any component or devicethat has a higher temperature than the heat spreader or lid duringoperation or otherwise provides or transfer heat to the lid or heatspreader regardless of whether the heat is generated by the heat sourceor merely transferred through or via the heat source.

Conventional polymeric thermal interface materials may be used as theTIM1. But the inventors hereof have recognized that currently usedpolymeric TIM1 materials are typically cure in place silicone gelmaterials that are required to be shipped and stored frozen. They alsohave short pot lives upon opening, short shelf lives, and requirespecial dispensing equipment to apply. After recognizing thesedrawbacks, the inventors hereof have developed and disclose hereinexemplary embodiments that eliminate, avoid or at least reduce theseaforementioned drawbacks associated with conventional polymeric TIM1materials.

As disclosed herein, exemplary embodiments include a TIM1 in the form ofa pad of thermoplastic material (e.g., thermoplastic phase changematerial, etc.) that may or may not be naturally tacky. In someembodiments, the TIM1 may have a softening temperature (e.g., a meltingtemperature, state transition or phase change temperature, etc.) higherthan a normal operating temperature of a heat source such as a CPU(e.g., normal operating temperature from about 60° C. to 100° C. or fromabout 30° C. to 40° C., etc.). In these exemplary embodiments, the padof thermoplastic material will soften or melt once (e.g., during anadhesive curing stage, during an initial operation of the CPU, etc.) andthen solidify. Thereafter, the pad of thermoplastic material may be usedbelow its softening or melting temperature and remain solidified. Inother exemplary embodiments, the TIM1 may have a softening less than orwithin an normal operating temperature range of a heat source such as aCPU.

In some exemplary embodiments, the TIM1 comprises a thermoplastic phasechange material having a softening temperature (e.g., melting point orphase change temperature, etc.) that falls within a range from about 75°C. to about 200° C. or about 125° C. to about 175° C., etc. Or, forexample, the TIM1 may have a softening or melting temperature of about40° C., 50° C., 75° C., etc. The TIM1 may have a thermal conductivity ofabout 0.3 Watts per meter per Kelvin (W/mK) or more, 3 W/mk or more, 5W/mk or more, etc., which thermal conductivity may be enhanced byincorporating thermally-conductive filler into the thermoplasticmaterial. In exemplary embodiments, the TIM1 may comprise a low meltingalloy having a melting temperature of about 160° C. or less.

Conventionally, it is common for an integrated heat spreader (IHS) orlid to be attached to a CPU and held in place via an adhesive along theoutside edges or perimeter rim of the IHS. The adhesive may be curedunder pressure (e.g., at a pressure that falls within a range of about 5pounds per square inch (psi) to about 100 psi or from about 10 psi toabout 50 psi, etc.) at a temperature (e.g., a temperature within a rangefrom about 75° C. to about 200° C. or from about 125° C. to about 175°C., a temperature of about 40° C., 50° C., 75° C., etc.). In exemplaryembodiments disclosed herein, the TIM1 material has a softeningtemperature (e.g., melting temperature, etc.) within a range from about75° C. to about 200° C. or from about 125° C. to about 175° C., or atemperature of about 40° C., 50° C., 75° C., etc. This allows the TIM1to soften (e.g., melt, change phase, become flowable, etc.) and flowduring the adhesive curing step. In these exemplary embodiments, theTIM1 (e.g., a thermoplastic pad, etc.) may be placed between the IHS (orlid) and a CPU prior to the adhesive curing step. The thermoplastic padsoftens, melts, or becomes flowable such that it flows to a thin bondline (e.g., having a thickness of about 10 mils or less, less than about5 mils, or from about 1 to about 3 mils, etc.) while under pressureduring the adhesive curing step, thereby resulting in a low thermalresistance (e.g., about 0.2° C. cm²/W, less than 0.15° C. cm²/W, etc.)joint or interface between the IHS and CPU. In alternative embodiments,the adhesive may not necessarily be cured under pressure. For example,mechanical stops may be used, and pressure may be used to squeeze theadhesive and the TIM1 to the desired degree. Then, the adhesive may becured at a temperature where the curing is not under pressure. In stillother embodiments, an integrated heat spreader (or lid) and TIM1 mayalso be attached and used with a CPU or other electronic device withoutusing any adhesive, such as by using gaskets and mechanical fasteners.

Exemplary embodiments disclosed herein may provide one or more (but notnecessarily any or all) of the following advantages. For example, theTIM1 may be pre-applied to the integrated heat spreader or lid, therebyreducing the number of assembly steps. The TIM1 may be naturally tackysuch that when pre-applied it will adhere to the heat spreader or lidwithout any additional adhesive needed (although adhesives could also beused). The heat spreader or lid may be preheated, and then the TIM1 maybe pre-applied to the warm heat spreader or lid. The TIM1 may besilicone-free, e.g., not have any detectable silicone, be entirely freeof silicone, etc. The TIM1 may be easy to rework. The TIM1 may be storedat room temperature, such that it doesn't have a pot life, does not haveto be warmed up before use, and has no moisture contamination.

A TIM1 disclosed herein may increase shelf life from 6 months or lessfor current products to 12 months or more. Exemplary embodiments alsoallow for the elimination of the need to ship and store material frozen,the elimination of pot life, and/or the elimination of the need fordispensing equipment providing additional floor space, while alsoproviding high thermal conductivity and low thermal resistance.

During thermal cycling, a cured TIM1 may delaminate from the edges ofthe CPU or IHS. If this delamination occurs, the interfacial contactresistance and thermal resistance of the TIM1 will increase greatly.This, in turn, may result in overheating of the CPU. To avoid thisdelamination and CPU overheating problem, exemplary embodimentsdisclosed herein include a TIM1 (e.g., thermoplastic, etc.). If the TIM1delaminates during thermal cycling, the CPU may then start to heat updue to the increased interfacial contact resistance and thermalresistance associated with the delaminated TIM1. Due to the heat fromthe operating CPU, the TIM1 will soften, which reduces the contactresistance and rewets the surfaces thereby resulting in reestablishmentor restoration of the delaminated joint and maintaining lower CPUtemperatures.

With reference now to the figures, FIG. 1 illustrates an exemplaryembodiment of an electronic device 100 having a TIM1 or thermalinterface material 104 embodying one or more aspects of the presentdisclosure. As shown in FIG. 1, the TIM1 or thermal interface material104 is positioned between a heat spreader or lid 108 and a heat source112, which may comprise one or more heat generating components ordevices (e.g., a CPU, die within underfill, semiconductor device, flipchip device, graphics processing unit (GPU), digital signal processor(DSP), multiprocessor system, integrated circuit, multi-core processor,etc.). A TIM2 or thermal interface material 116 is positioned between aheat sink 120 and the heat spreader or lid 108.

By way of example, the heat source 112 may comprise a central processingunit (CPU) or processor die mounted on a printed circuit board (PCB)124. The PCB 124 may be made of FR4 (flame retardant fiberglassreinforced epoxy laminates) or other suitable material. Also in thisexample, the heat spreader or lid 108 is an integrated heat spreader(IHS), which may comprise metal or other thermally-conductive structure.The heat spreader or lid 108 includes a perimeter ridge, flange, orsidewall portions 128. Adhesive 132 is applied to and along theperimeter ridge 128 for attaching the heat spreader or lid 108 to thePCB 124. The perimeter ridge 128 may thus protrude sufficiently downwardto extend around the silicone die on the PCB 124 and thereby allowcontact between the adhesive 132 on the perimeter ridge 128 and the PCB124. Advantageously, adhesively attaching the heat spreader or lid 108to the PCB 124 may also help stiffen the package, which is attached tothe base PCB.

Also shown in FIG. 1 are pin connectors 136. The heat sink 120 generallyincludes a base from which outwardly protrude a series of fins.Alternative embodiments may include a TIM1 being used with differentelectronic devices besides what is shown in FIG. 1, different heatgenerating components besides a CPU or processor die, different heatspreaders, and/or different heat sinks. Accordingly, aspects of thepresent disclosure should not be limited to use with any single type ofelectronic device as exemplary embodiments may include a TIM1 that isusable with any of a wide range of electronic devices, heat sources, andheat spreaders.

The TIM2 or thermal interface material 116 may comprise any of a widerange of thermal interface materials including thermal interfacematerials from Laird Technologies, Inc. (e.g., Tflex™ 300 series thermalgap filler materials, Tflex™ 600 series thermal gap filler materials,Tpcm™ 580 series phase change materials, TpIi™ 200 series gap fillers,and/or Tgrease™ 880 series thermal greases from Laird Technologies, Inc.of Saint Louis, Mo., etc.).

The TIM1 or thermal interface material 116 may also comprise a widerange of materials, such as phase change and/or thermoplastic thermalinterface materials. In some exemplary embodiments, the TIM1 comprises apad of thermoplastic and/or phase change material having a softeningpoint (e.g., a melting temperature, phase change temperature, etc.)higher than the normal operating temperature of the heat source 112(e.g., a CPU having a normal operating temperature from about 60° C. to100° C., etc.). For example, the TIM1 may have a softening temperatureof about 120° C., and the normal operating temperature of the CPU orother heat source may be about 115° C. The TIM1 pad will soften or meltonce (e.g., during an adhesive curing stage, during an initial operationof the CPU, etc.) and then solidify. Thereafter, the TIM1 pad ofthermoplastic and/or phase change material may be used below itssoftening or melting temperature and remain solidified. In someexemplary embodiments, the TIM1 may comprise a thermal interfacematerial including a thermally reversible gel as disclosed hereinafterand in U.S. Patent Application Publication No. US 2011/0204280, theentire disclosure of which is incorporated herein by reference in itsentirety.

In other exemplary embodiments, the TIM1 may comprise a pad ofthermoplastic and/or phase change material having a softening point(e.g., a melting temperature, phase change temperature, etc.) that isless than or within a normal operating temperature range of the heatsource 112 (e.g., CPU having a normal operating temperature range fromabout 60° C. to 100° C., etc.). IceKap™ thermally-conductive elastomericoil-gel based interface material from Laird Technologies, Inc. is anexample of a TIM1 that may be used in exemplary embodiments.

FIG. 2 shows a TIM1 or thermal interface material 204 on a portion 240of a heat spreader or lid 208. In this example, the heat spreader or lid208 may be an integrated heat spreader. The heat spreader or lid 208 maybe positioned relative to (e.g., on top of, etc.) the heat source suchthat the TIM1 or thermal interface material 204 is sandwiched betweenthe heat spreader or lid 208 and the heat source, with the TIM1compressed against the heat source. The heat source may comprise one ormore heat generating components or devices (e.g., a CPU, die withinunderfill, semiconductor device, flip chip device, graphics processingunit (GPU), digital signal processor (DSP), multiprocessor system,integrated circuit, multi-core processor, etc.).

With continued reference to FIG. 2, the heat spreader or lid 208includes a perimeter ridge or flange 228 about the generally flat,planar portion 240. Adhesive may be applied to and along the perimeterridge 228 for attaching the heat spreader or lid 208 to a PCB. Theperimeter ridge 228 may thus protrude sufficiently outward from theportion 240 to extend around an electronic component mounted on a PCBand thereby allow contact between the adhesive on the perimeter ridge228 and the PCB.

Adhesively attaching the heat spreader or lid 208 to the PCB may alsohelp stiffen the package, which is attached to the base PCB. The packageitself typically includes a mini PCB with a chip and the heat spreaderor lid 208.

Alternative embodiments may include other ways or means of attaching thelid or heat spreader to the PCB. For example, adhesive may be disposedalong less than all sides of the perimeter of the lid or heat spreader.Or, for example, the lid or heat spreader may be a flat plate withoutany perimeter ridge or sidewalls. In which case, the adhesive itself maybridge the gap between the flat lid and the PCB. Accordingly, aspects ofthe present disclosure are not limited to any particular attachmentmethod between the lid or heat spreader and the PCB.

Also disclosed herein are exemplary embodiments of methods relating toor establishing of a thermal joint, interface, or pathway for conductingheat between a heat spreader and a heat source (e.g., one or more heatgenerating components, etc.) using a phase change and/or thermoplasticthermal interface material (TIM1). In an exemplary embodiment, a methodgenerally includes positioning a thermal interface material (TIM1)(e.g., a free-standing thermoplastic phase change pad, etc.) on asurface of a heat spreader before attachment to an electronic component(e.g., prior to an adhesive curing process, etc.). In this example, theTIM1 may have a softening temperature (e.g., melting temperature, phasechange temperature, temperature at which the material hardnessdecreases, etc.) higher than a normal operating temperature of theelectronic component. Or, for example, the TIM1 may have a softeningtemperature that is less than or within a normal operating temperaturerange of the electronic component. The softening temperature of the TIM1may be low enough such that the TIM1 will soften, melt, and flow duringan adhesive curing process (e.g., when an adhesive is cured underpressure at a temperature of between 100° C. to 200° C. or from 125° C.to 175° C., etc.). During the adhesive curing process, the TIM1 willflow to a thin bond line whereby the TIM1 creates a relatively shortheat path with low thermal resistance between the heat spreader and theelectronic component.

After the curing process, the TIM1 solidifies and forms a low thermalresistance thermal joint/pathway between the electronic component andthe heat spreader. In some exemplary embodiments, the TIM1 has asoftening or melting temperature above the normal operating temperatureof the electronic component, such that the electrical component will notreach a high enough operating temperature to deform, soften, or melt theTIM1. The solidified thermal joint precludes the electronic componentfrom heating beyond its normal operating temperature upon subsequentoperation. The TIM1 may be selected so as to deform only during theinitial adhesive curing phase so as to avoid problems of liquefaction.

In another exemplary embodiment, a method generally includes attaching aheat spreader having a TIM1 thereon to an electronic component (e.g.,CPU, etc.) by curing an adhesive. The TIM1 melts/softens and flows to athin bond line while under pressure during the curing process, whichresults in a thermal joint/pathway having low thermal resistance betweenthe electronic component and the heat spreader. The TIM1 may have asoftening point less than, within, or higher than a normal operatingtemperature range of the electronic component.

In some exemplary embodiments, the method may further includeestablishing a thermal joint between the heat spreader and a heat sinkby positioning a thermal interface material (TIM2) between the heat sinkand the heat spreader. A thermally conductive heat path may then beestablished from the heat source through the TIM1, the heat spreader,and the TIM2 to the heat sink such that heat from the heat source (e.g.,generated by one or more heat generating components, etc.) istransferrable to the heat sink via the TIM1, the heat spreader, and theTIM2. If the heat source is a semiconductor device, for example, thenthe semiconductor device would be in effective thermal communicationwith the heat sink via the TIM1, the heat spreader, and TIM2.

In an alternative exemplary embodiment, the TIM1 may comprise a coatingor material that is coated or otherwise applied (e.g., by screenprinting, stenciling, etc.) onto a heat spreader or lid. The heatspreader may then be positioned relative to the heat source (e.g., oneor more heat generating components, etc.) such that the TIM1 is betweenthe heat spreader and the heat source. The TIM1 is initially in a solidstate and may not fill all the resulting voids between the matingsurfaces of the heat spreader and heat source. Thus, during initialoperation, the thermal path may be an inefficient one. This inefficiencymay cause the heat source to reach a temperature above a normaloperating temperature range and/or above the softening temperature(e.g., melting temperature, etc.) of the TIM1. For example, one or moreheat generating components while operating may heat the TIM1 to or aboveits softening temperature such that the TIM1 becomes flowable to a thinbond line, and fill the voids between the mating surfaces of the heatsource and the heat spreader. This creates an efficient thermal jointhaving low thermal resistance. In turn, more heat flows from the heatsource to the heat spreader such that the temperature is reduced to anormal operating temperature. During this cool down, the TIM1temperature drops below its softening temperature, which returns theTIM1 to its solid state (e.g., pad, etc.) while the previouslyestablished thermal joint is maintained. Upon subsequent operation, thenormal operating temperature will not be exceeded as the previouslyestablished thermal joint conducts heat from the heat source to the heatspreader. The TIM1 will not melt or flow as the normal operatingtemperature remains below the temperature at which TIM1 melts or becomesflowable. As the TIM1 does not melt and flow, the TIM1 may thus maintainthe thermal conductivity of its solid state, which may be higher thanthe thermal conductivity of its liquid state. Moreover, as the TIM1 willnot flow away from the thermal joint, the joint integrity is maintained.

In some exemplary embodiments, the TIM1 may comprise a thermal interfacematerial including a thermally reversible gel as disclosed in U.S.Patent Application Publication No. US 2011/0204280, the entiredisclosure of which is incorporated herein by reference in its entirety.In an exemplary embodiment, the TIM1 includes at least one thermallyconductive filler (e.g., boron nitride, alumina, and zinc oxide, etc.)in a thermally reversible gel (e.g., oil gel, etc.). The thermallyreversible gel comprises a gelling agent and an oil and/or solvent. Theoil and/or solvent may comprise paraffinic oil and/or solvent. Thegelling agent may comprise a thermoplastic material. The thermoplasticmaterial may comprise a styrenic block copolymer. The thermal interfacematerial may be an oil gel that includes paraffinic oil and di-blockand/or tri-block styrenic copolymers. The TIM1 may include naphthenicoils and solvents and/or paraffinic oils and solvents (e.g., isopars, ahigh temperature stable oil and/or solvent, etc.). Thermoplasticmaterials (e.g., thermoplastic elastomers, etc.) may be used for thegelling agent of the oil gel. Suitable thermoplastic materials includeblock copolymers, such as di-block and tri-block polymers (e.g.,di-block and tri-block styrenic polymers, etc.). A di-block containingpad will be relatively soft at room temperature, which tends to beimportant because most assembly or installation is performed at roomtemperature and a softer di-block containing pad will advantageouslyreduce the assembly pressures generated. In some embodiments, the TIM1may include an oil gel resin system in which the oil gel is formulatedto soften at a temperature higher or less than 150 degrees Celsius, suchas within a temperature range from about 5 degrees Celsius to about 200degrees Celsius.

One or more thermally conductive fillers may be added to create athermally conductive interface material in which one or more thermallyconductive fillers will be suspended in, added to, mixed into, etc. thethermally reversible gel. For example, at least one thermally conductivefiller may be added to a mixture including gellable fluid and gellingagent before the gellable fluid and gelling agent have gelled or formthe thermally reversible gel. As another example, at least one thermallyconductive filler may be added to the gellable fluid and then gellingagent may be added to the mixture containing gellable fluid andthermally conductive filler. In yet another example, at least onethermally conductive filler may be added to the gelling agent and thengellable fluid may be added to the mixture containing gelling agent andthermally conductive filler. By way of further example, at least onethermally conductive filler may be added after the gellable fluid andgelling agent have gelled. For example, at least one thermallyconductive filler may be added to the gel when the gel may be cooled andbe loosely networked such that filler can be added. The amount ofthermally conductive filler in the thermally reversible gel may vary indifferent embodiment. By way of example, some exemplary embodiments of athermal interface material may include not less than 5 percent but notmore than 98 percent by weight of at least one thermally conductivefiller.

In exemplary embodiments, the TIM1 may comprise a thermally conductiveelastomeric interface material. By way of example, an exemplaryembodiment may include a TIM1 having the properties shown in the tableimmediately below. Additionally, or alternatively, an exemplaryembodiment may include a TIM1 having properties such as low contactresistance, easy to flow to thin bond lines, ability to wet multiplesurfaces, etc.

Typical Property Description Test Method Color Grey VisualConstruction/Composition Non-reinforced film Specific Gravity, g/cc 2.51 Helium Pycnometer Minimum bond line 0.025 (1) Laird Test Methodthickness, mm (mils) Thermal conductivity, W/mK  4.7 Hot Disk ThermalConstants Analyzer Thermal Resistance, 0.064 (0.010) ASTM D5470 ° C.cm²/W (° C. in²/W) Available Thicknesses, 0.125-0.625 (5.25) Laird TestMethod mm (mils) Room Temperature Hardness, 85 ASTM D2240 shore 00Volume Resistivity, ohm-cm 10¹⁵ ASTM D257

For lower power lower operating temperature systems (e.g., 30° C., 40°C., etc.), exemplary embodiments may include a TIM1 that comprises (orhas properties similar to) a Tpcm™ 780 phase change thermal interfacematerial from Laird Technologies, Inc. of Saint Louis, Mo., and,accordingly, have been identified by reference to trademarks of LairdTechnologies, Inc. Details on these different materials are available atwww.lairdtech.com. In such exemplary embodiments, the TIM1 may have theproperties shown in the table immediately below. At a temperature of 70°C., the TIM1 may have bond line thickness of about 0.0015 inches at 20psi, of about 0.001 inches at 40 psi, of about 0.005 inches at 100 psi,etc.

PROPERTIES Tpcm ™ 760 TEST METHOD Color Grey Visual Thickness, inches(mm) 0.016″ (0.406) 0.025″ (0.635) Thickness Tolerance, ±0.0016″(0.0406) inches (mm) ±0.0025 (0.0635) Construction & Non reinforced filmComposition Specific Gravity, g/cc  2.48 Helium Pycncmeter Place ChangeSoftening ~45° C. to 70° C. Hange, ° C. Thermal Conductivity,  5.4 HotDisk Thermal W/mK Constants Analyser Hardness (Shore 00) 85 ASTM D2240 3sec @ 21 C. Thermal Resistance 0.025 (0.004) ASTM D5470 (modified) 70°C., 845 kPa, ° C · cm²/W (50 psi, ° C. · in²/W) Outgassing TML 0.51%ASTM E595 Outgassing CVCM 0.20% ASTM E595

In exemplary embodiments, the TIM1 is engineered so it does notdrastically change phase within its operating temperature range. Forexample, the TIM1 may not significantly soften or change phase untilabove the normal operating temperature of the component(s) to be cooled.In some exemplary embodiments, the TIM1 may having a softening point(e.g., a melting temperature, phase change temperature, etc.) that isless than or within a normal operating temperature range of the heatsource. For example, the TIM1 may have a softening temperature rangefrom about 45° C. to about 70° C., while the heat source may have anormal operating temperature of about 80° C. or above.

FIG. 3 is a line graph showing durometer shore 00 test results versustemperature for a TIM1 that may be used in exemplary embodiments. Thesetest results generally show that the TIM1 maintains significantstructure within its entire intended use temperature. The test resultsalso show that the tested TIM1 is relatively soft at room temperatureand softens with increasing temperature but remain generally solidwithin the operating temperature range. Immediately below is a table ofthe two durometer test results (shore 00 for 3 seconds) for the TIM 1and also showing the average of the two tests, which averages wereplotted in FIG. 3.

Temperature ° C. Test 1 Test 2 Average 25 78.2 79.7 78.95 50 75.5 78.577.00 75 60.4 56.8 58.60 100 57.1 53.4 55.25 125 37.8 45.2 41.50 15025.9 32.6 29.25

The tables above list exemplary thermal interface materials that havethermal conductivities of 4.7 and 5.4 W/mK. These thermal conductivitiesare only examples as other embodiments may include a thermal interfacematerial with a thermal conductivity higher than 5.4 W/mK, less than 4.7W/mK, or other values. For example, some embodiments may include athermal interface material having a thermal conductivity higher thanair's thermal conductivity of 0.024 W/mK, such as a thermal conductivityof about 0.3 W/mk, of about 3.0 W/mK, or somewhere between 0.3 W/mk and3.0 W/mk, etc.

A wide range of different thermally conductive fillers may be used inexemplary embodiments. In some exemplary embodiments, the thermallyconductive fillers have a thermal conductivity of at least 1 W/mK (Wattsper meter-Kelvin) or more, such as a copper filler having thermallyconductivity up to several hundred W/mK, etc. Suitable thermallyconductive fillers include, for example, zinc oxide, boron nitride,alumina, aluminum, graphite, ceramics, combinations thereof (e.g.,alumina and zinc oxide, etc.). In addition, exemplary embodiments of athermal interface material may also include different grades (e.g.,different sizes, different purities, different shapes, etc.) of the same(or different) thermally conductive fillers. For example, a thermalinterface material may include two different sizes of boron nitride. Byvarying the types and grades of thermally conductive fillers, the finalcharacteristics of the thermal interface material (e.g., thermalconductivity, cost, hardness, etc.) may be varied as desired.

In alternative exemplary embodiments, the TIM1 may be a multilayeredthermal interface material that may comprise a heat spreader (e.g., aninterior heat spreading core formed from metal, metal alloy, graphite,sheet of stamped aluminum or copper, etc.) that is isotropic oranisotropic. The heat spreader may be disposed within or sandwichedbetween layers of a thermoplastic thermal interface material. Or, forexample, a thermoplastic thermal interface material may be applied to(e.g., coated onto, etc.) the heat spreader on or along one or bothsides.

The size of the TIM1 relative to the footprint of the heat source, e.g.,component(s), to be cooled may vary depending on the particularapplication. The TIM1 may have a larger, smaller, or about equalfootprint size as that of the footprint of the heat source, e.g.,components, to be cooled. For example, the TIM1 may be initially sizedsuch that it has a footprint smaller than that of the componentfootprint. But the TIM1 may be configured to have a have a greaterinitial thickness so that the volume of the TIM1 material is essentiallythe same as that of a thinner pad with a footprint about the same sizeas the component(s) to be cooled. When the TIM1 is heated to atemperature at which it become flowable, the TIM1 would flow to form athin bond line as disclosed herein, which, in turn, would also increasethe footprint of the TIM1.

The TIM1 may be applied using a variety of methods. For example, theTIM1 may be pre-applied to the integrated heat spreader or lid, therebyreducing the number of assembly steps. The TIM1 may be naturally tackysuch that when pre-applied it will adhere to the heat spreader or lidwithout any additional adhesive needed (although adhesives could also beused). As another example, the heat spreader or lid may be preheated,and then the TIM1 may be pre-applied to the warm heat spreader or lid.The TIM1 may also be pre-applied to the component(s) to be cooledinstead of the heat spreader or lid.

In another exemplary embodiment, the TIM1 may be added into or otherwisebe present in a solvent. The solvent and TIM1 may be applied as a greaseor dispensed material to a heat spreader or lid or to the one or morecomponent(s) to be cooled. The assembly may then be put together, suchas by adhesively attaching the heat spreader or lid to the PCB thatincludes the one or more components to be cooled. The solvent may thenbe allowed to slowly evaporate. After the solvent has evaporated, theTIM1 would remain, which would have a softening temperature above,within, or less than the normal operating temperature range of the oneor more component(s). In this particular example, the softening ormelting step of TIM1 during lid/heat spreader attachment would not berequired in this example. Because the TIM1 is assembled at lowviscosity, the TIM1 would fill voids and wet the surfaces.

In exemplary embodiments that include a TIM2, a wide variety ofmaterials may be used for the TIM2. In exemplary embodiments, the TIM2may include compliant or conformable silicone pads, non-silicone basedmaterials (e.g., non-silicone based gap filler materials, thermoplasticand/or thermoset polymeric, elastomeric materials, etc.), silk screenedmaterials, polyurethane foams or gels, thermal putties, thermal greases,thermally-conductive additives, etc. In exemplary embodiments, the TIM2may be configured to have sufficient conformability, compliability,and/or softness to allow the TIM2 material to closely conform to amating surface when placed in contact with the mating surface, includinga non-flat, curved, or uneven mating surface. By way of example, someexemplary embodiments include an electrically conductive soft thermalinterface material formed from elastomer and at least onethermally-conductive metal, boron nitride, and/or ceramic filler, suchthat the soft thermal interface material is conformable even withoutundergoing a phase change or reflow. The TIM2 may include one or more ofTflex™ 300 series thermal gap filler materials, Tflex™ 600 seriesthermal gap filler materials, Tpcm™ 580 series phase change materials,TpIi™ 200 series gap fillers, and/or Tgrease™ 880 series thermal greasesfrom Laird Technologies, Inc. of Saint Louis, Mo., and, accordingly,have been identified by reference to trademarks of Laird Technologies,Inc. Details on these different materials are available atwww.lairdtech.com. Other thermally-conductive compliant materials orthermally-conductive interface materials can also be used for the TIM2.For example, the TIM2 may comprise graphite, a flexible graphite sheet,exfoliated graphite and/or compressed particles of exfoliated graphite,formed from intercalating and exfoliating graphite flakes, such aseGraf™ commercially available from Advanced Energy Technology Inc. ofLakewood, Ohio. Such intercalating and exfoliating graphite may beprocessed to form a flexible graphite sheet, which may include anadhesive layer thereon.

In some exemplary embodiments, a method of establishing a thermal jointfor conducting heat between a heat spreader and a heat source of anelectronic device generally includes positioning a thermal interfacematerial (TIM1) between the heat spreader and the heat source. Thethermal interface material may comprise a phase change thermal interfacematerial having a softening temperature that is below or within a normaloperating temperature range of the heat source. For example, the phasechange thermal interface material may have a phase change temperaturebelow or within the normal operating temperature range of the heatsource. The phase change thermal interface material may also be shearthinning and thixotropic such that the phase change thermal interfacematerial is not flowable at the phase change temperature except underpressure. In which case, the phase change thermal interface material mayinclude the proper combination of particles, additives, and polymersthat results in a material that maintains shape unless force is appliedeven when in its softened state. Also, for example, the phase changethermal interface material may comprise a phase change material (e.g., asilicone-free or silicone wax phase change material, phase changematerial having silicone additives, etc.) having a softening temperaturerange from about 45 degrees Celsius to about 70 degrees Celsius.

The phase change thermal interface material may have a phase changetemperature above the normal operating temperature range of the heatsource, such that the phase change thermal interface material softens,without melting, within the normal operating temperature range of theheat source. The method may include heating the phase change thermalinterface material to a temperature above the normal operatingtemperature range, such that the phase change thermal interface materialis flowable under pressure; and allowing the phase change thermalinterface material to return to a temperature below or within the normaloperating temperature range, whereby the phase change thermal interfacematerial establishes a thermal joint between the heat spreader and theheat source. The method may include heating the phase change thermalinterface material to a phase change temperature while under pressuresuch that the phase change thermal interface material flows to form athin bond line between the heat spreader and the heat source; andallowing the phase change thermal interface material to return to asolid state, whereby the phase change thermal interface materialestablishes a thermal joint between the heat spreader and the heatsource. The method may further include applying the phase change thermalinterface material to the heat spreader before positioning the phasechange thermal interface material between the heat spreader and the heatsource; or applying the phase change thermal interface material to theheat source before positioning the phase change thermal interfacematerial between the heat spreader and the heat source. The method mayinclude curing an adhesive for attaching the heat spreader to theelectronic device, which curing process also heats the phase changethermal interface material to at least the softening temperature.

In some exemplary embodiments, a method of establishing a thermal jointfor conducting heat between a heat spreader and a heat source of anelectronic device generally includes positioning a thermal interfacematerial (TIM1) between the heat spreader and the heat source. Dependingon the particular embodiment, the thermal interface material may beoperable above, below, or within a normal operating temperature range ofthe heat source for reestablishing or restoring a thermal joint orthermal path after a loss of contact between the thermal interfacematerial and another component (e.g., heat spreader, heat source, etc.)resulting in poor thermal transfer, etc. For example, if delamination ofthe thermal interface material occurs during thermal cycling, theninterfacial contact resistance and thermal resistance of the thermaljoint will increase whereby heat from the heat source will cause thethermal interface material to soften, reduce contact resistance, andrewet the surfaces. With time, the softened thermal interface materialmay then restore or reestablish the thermal joint and improve thermaltransfer, e.g., back to the original thermal transfer, etc. As anotherexample, small voids (e.g., voids created by outgassing, etc.) in thethermal interface material may become smaller and totally fill in whenthe thermal interface material is under pressure (e.g., constantpressure, etc.) over time.

In some exemplary embodiments, a method generally includes positioning athermal interface material (TIM1) between a heat spreader and a heatsource of an electronic device prior to curing an adhesive for attachingthe heat spreader to the electronic device. The thermal interfacematerial has a softening temperature that is below or within a normaloperating temperature range of the heat source. The method may furtherinclude attaching the heat spreader to the electronic device by curingan adhesive. During the curing, the thermal interface material may beheated under pressure such that the thermal interface material flows toform a thin bond line between the heat spreader and the heat source. Themethod may also include allowing the thermal interface material toreturn to a solid state, whereby the thermal interface materialestablishes a thermal joint having low thermal resistance between theheat spreader and the heat source. The thermal interface material may beoperable for reestablishing or restoring the thermal joint as describedherein.

In a further exemplary embodiment, an electronic device generallyincludes a lid and a semiconductor device having a normal operatingtemperature range. A first thermal interface material (TIM1) establishesa restorable thermal joint between the lid and the semiconductor device.The first thermal interface material may comprise a phase change thermalinterface material having a softening temperature that is below orwithin a normal operating temperature range, and/or the first thermalinterface material may be operable for reestablishing or restoring thethermal joint between the lid and the semiconductor device. Theelectronic device may also include a heat sink. A second thermalinterface material may be positioned between the lid and the heat sink.The semiconductor device may be in effective thermal communication withthe heat sink via the first thermal interface material, the lid, and thesecond thermal interface material.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms, and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail. In addition, advantages and improvements that maybe achieved with one or more exemplary embodiments of the presentdisclosure are provided for purpose of illustration only and do notlimit the scope of the present disclosure, as exemplary embodimentsdisclosed herein may provide all or none of the above mentionedadvantages and improvements and still fall within the scope of thepresent disclosure.

Specific dimensions, specific materials, and/or specific shapesdisclosed herein are example in nature and do not limit the scope of thepresent disclosure. The disclosure herein of particular values andparticular ranges of values for given parameters are not exclusive ofother values and ranges of values that may be useful in one or more ofthe examples disclosed herein. Moreover, it is envisioned that any twoparticular values for a specific parameter stated herein may define theendpoints of a range of values that may be suitable for the givenparameter (i.e., the disclosure of a first value and a second value fora given parameter can be interpreted as disclosing that any valuebetween the first and second values could also be employed for the givenparameter). For example, if Parameter X is exemplified herein to havevalue A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, and 3-9.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

The term “about” when applied to values indicates that the calculationor the measurement allows some slight imprecision in the value (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If, for some reason, the imprecisionprovided by “about” is not otherwise understood in the art with thisordinary meaning, then “about” as used herein indicates at leastvariations that may arise from ordinary methods of measuring or usingsuch parameters. For example, the terms “generally,” “about,” and“substantially,” may be used herein to mean within manufacturingtolerances. Or, for example, the term “about” as used herein whenmodifying a quantity of an ingredient or reactant of the invention oremployed refers to variation in the numerical quantity that can happenthrough typical measuring and handling procedures used, for example,when making concentrates or solutions in the real world throughinadvertent error in these procedures; through differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods; and the like. The term “about”also encompasses amounts that differ due to different equilibriumconditions for a composition resulting from a particular initialmixture. Whether or not modified by the term “about,” the claims includeequivalents to the quantities.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements, intended orstated uses, or features of a particular embodiment are generally notlimited to that particular embodiment, but, where applicable, areinterchangeable and can be used in a selected embodiment, even if notspecifically shown or described. The same may also be varied in manyways. Such variations are not to be regarded as a departure from thedisclosure, and all such modifications are intended to be includedwithin the scope of the disclosure.

What is claimed is:
 1. A method of establishing a thermal joint forconducting heat between a heat spreader and a heat source of anelectronic device, the heat source having a normal operating temperaturerange, the method comprising: positioning a thermal interface material(TIM1) in an initial solid state in the form of a pad between the heatspreader and the heat source, wherein the thermal interface materialcomprises a phase change thermal interface material; and heating thethermal interface material, while under pressure, to a temperaturewithin a range from about 45 degrees Celsius to about 70 degrees Celsiussuch that the thermal interface material becomes flowable to form a thinbond line between the heat spreader and the heat source and establishesthe thermal joint between the heat spreader and the heat source,whereupon removal of the pressure, the thermal interface materialreturns to the solid state and remains solidified as the thermalinterface material precludes the heat source from heating beyond thenormal operating temperature range during operation of the electronicdevice and heat is flowable through the thermal joint from the heatsource to the heat spreader during operation of the electronic devicewhereby temperature of the heat source is reduced; wherein the thermalinterface material is silicone free; and wherein the phase changethermal interface material is shear thinning and thixotropic such thatthe phase change thermal interface material is not flowable at thetemperature within the range from about 45 degrees Celsius to about 70degrees Celsius except under pressure and such that the phase changethermal interface material maintains shape unless force is applied evenwhen the phase change thermal interface material is at the temperaturewithin the range from about 45 degrees Celsius to about 70 degreesCelsius.
 2. The method of claim 1, wherein: the thermal interfacematerial is soft at room temperature with a shore 00 hardness of lessthan 80 at a temperature of 25 degrees Celsius; and the normal operatingtemperature range of the heat source is within a range from about 30degrees Celsius to about 60 degrees Celsius.
 3. The method of claim 1,wherein: the thermal interface material is soft at room temperature witha shore 00 hardness of less than 80 at a temperature of 25 degreesCelsius; and the heating the thermal interface material, while underpressure, to a temperature within a range from about 45 degrees Celsiusto about 70 degrees Celsius comprises heating the thermal interfacematerial, while under pressure, to a temperature that is below thenormal operating temperature range of the heat source.
 4. The method ofclaim 1, wherein: the thermal interface material is soft at roomtemperature with a shore 00 hardness of less than 80 at a temperature of25 degrees Celsius; and the heating the thermal interface material,while under pressure, to a temperature within a range from about 45degrees Celsius to about 70 degrees Celsius comprises heating thethermal interface material, while under pressure, to a temperature thatis within the normal operating temperature range of the heat source. 5.The method of claim 1, wherein: the thermal interface material is softat room temperature with a shore 00 hardness of less than 80 at atemperature of 25 degrees Celsius; and the heating the thermal interfacematerial, while under pressure, to a temperature within a range fromabout 45 degrees Celsius to about 70 degrees Celsius comprises heatingthe thermal interface material, while under pressure, to a temperatureof about 50 degrees.
 6. The method of claim 1, wherein: the phase changethermal interface material is naturally tacky; and the method furthercomprises pre-applying the phase change thermal interface material tothe heat spreader, without using any additional adhesive between thephase change thermal interface material and the heat spreader, beforepositioning the phase change thermal interface material between the heatspreader and the heat source, such that the phase change thermalinterface material adheres to the heat spreader without any additionaladhesive.
 7. The method of claim 1, wherein the method includes curingan adhesive for attaching the heat spreader to the electronic device,which curing heats the phase change thermal interface material to thetemperature within the range from about 45 degrees Celsius to about 70degrees Celsius.
 8. The method of claim 1, wherein: the electronicdevice comprises a semiconductor device having the heat source; the heatspreader comprises a lid of the electronic device; the phase changethermal interface material is positioned between the semiconductordevice and the lid; and the method further comprises positioning asecond thermal interface material (TIM2) between a heat sink and thelid, whereby the semiconductor device is in effective thermalcommunication with the heat sink via the phase change thermal interfacematerial, the lid, and the second thermal interface material.
 9. Themethod of claim 1, wherein the method comprises positioning the thermalinterface material between the heat spreader and the heat source priorto curing an adhesive for attaching the heat spreader to the electronicdevice.
 10. The method of claim 9, further comprising attaching the heatspreader to the electronic device by curing the adhesive.
 11. The methodof claim 9, further comprising attaching the heat spreader to theelectronic device by curing the adhesive, and wherein the heating thethermal interface material, while under pressure, to the temperaturewithin the range from about 45 degrees Celsius to about 70 degreesCelsius is during the curing.
 12. The method of claim 1, wherein: thethermal interface material has an initial footprint smaller than afootprint of the heat source; and the method includes increasing thefootprint of the thermal interface material during the heating of thethermal interface material while under pressure.
 13. The method of claim1, wherein the thermal interface material has a shore 00 hardness lessthan 80 at a temperature of 25 degrees Celsius.